The present invention relates to a combined milling and cleaning tool intended for use in downhole environments.
It is a common procedure during the completion of a well to line the newly drilled bore with casing or liner, the latter typically being used near the production area of the bore. Casing, which is usually made of heavy steel piping, is used to prevent collapse of newly drilled bore segments and contamination of the oil or gas reservoir contained therein. Typically the casing or liner is run into the bore from the surface and held in place by introducing cement between the external surface of the casing or liner and the internal surface of the wellbore, with each section of the bore being drilled with consecutively smaller drill bits and then lined with proportionately smaller casing or liner sections.
It will be appreciated that after cementing the casing or liner in place, it is often necessary to clean the interior of the casing or liner to remove obstructions such as burrs or lumps of cement which remain within the tubing after the cementing procedure. A commonly used cleaning tool, well known to the art, is a casing scraper which incorporates blades typically made of a resilient material such as steel. The blades are used to scrape the interior surface of the casing or liner. Milling tools are also well known in the art and are used to “dress off” the polished bore receptacle liner top in a new wellbore. Milling removes burrs, and grinds the polished bore receptacle to allow smooth and easy entry of subsequent tools through the liner. Milling tools are also commonly used to remove casing present in a wellbore if said casing is damaged in any way. Milling tools provide a cutting or grinding action and are necessarily formed from a material which is hard enough to cut or grind the liner top, which is a machine steel tube. Often, the tool is produced with carbide inserts as this material is hard enough to mill casing or liner steel.
Historically when completing a bore using a scraper and milling tool, the scraping tool is run into the wellbore on a work string to clean the interior of the casing. This first tool must then be removed or “tripped” from the bore before the milling tool can be run to tidy up or “dress off” the liner top. As a consequence, the cost and time taken to finish the bore is increased as it is necessary to perform two trips down the well.
Previous attempts to run milling tools and scrapers into a wellbore at the same time have encountered problems, as it is usual for the combined milling and scraping action to dislodge and create additional debris within the casing and liner. This is typically suspended in the well fluid in the bore and negates much of the cleaning which is carried out. It has therefore still been necessary when carrying out a combined operation to run a second trip down the well to clean the wellbore before production is commenced.
It would be very desirable to be able to run a cutting and a milling tool together in one operation, eliminating at least one trip into and out of the borehole to finish said bore, as the beginning of profitable production will not be delayed.
It is an object of the present invention to provide an improved tool for use when completing a downhole wellbore. In particular is an object of the present invention to provide a tool, which can carry out milling and scraping functions at the same time.